1. Field of the Invention
The present invention relates to a cover for a static or moving radar to be used for protecting it.
2. Description of the Prior Art
A radome is used as a protecting cover for a radar, and the performance requirements thereof depend upon whether the radome is used on the ground or a ship or in a missile or aircraft.
The role of a radome to be used on the ground or a ship is to protect a radar from the surrounding severe meteorological conditions. On the other hand, a missile or aircraft is generally equipped with a radar at the front, so that a radome to be used in a missile or aircraft must have excellent aerodynamic characteristics, and must, of course, withstand the severe atmospheric conditions.
In addition to these performance requirements, it is also important that a radome does not act as an electrical shield against radio waves which are essential to a radar system. In particular, the material of a radome is required to have an ability to transmit radio waves, and therefore is selected from those having low dielectric loss (tan .delta.) and low relative dielectric constant (.epsilon..sub.r) These properties serve to enhance the radio wave transmission of a radome.
In the prior art, studies on the material of a radome were made mainly from the standpoints of radio wave transmission properties, so that most of the radomes of the prior art were made of FRPs (fiber reinforced plastics), ceramics, resin foams or combinations of two of more of them. In particular, radomes to be used on the ground or a ship are generally made of FRPs with the main purpose of enhancing the weather resistance, while radomes to be used at the front of an aircraft or missile are generally made of ceramics or reinforced ceramics under the necessity of enhancing the heat resistance.
A radio wave transmitting material made by using a ceramic fiber has recently been disclosed (in Japanese Patent Laid-Open No. 173403/1988), but the point of this disclosure is also to use FRPs or FRCs (fiber reinforced ceramics).
Further, radomes made by using light-transmitting ceramics have also been disclosed for use in the field necessitating the transmission of light (Japanese Patent Laid-Open No. 241300/1991). However, the material usable in this case is severely restricted, because it is required to transmit not only radio waves but also light.
As described above, a radome has various uses. Radomes made of resins causes degradation owing to the poor weather resistance of the resins. On the other hand, ceramic radomes also have the following problems: ceramics themselves are difficult of machining; the ceramics used in producing radomes to be used on the ground or a ship must be improved in weather resistance, while those used in producing a radomes for an aircraft or missile must be improved in both weather resistance and machinability; and the usable frequency of a radar system depends on the relative dielectric constant (.epsilon..sub.r) of the ceramics used.